1. Field of the Invention
This invention relates to an aircraft proximity warning and collision avoidance system wherein a first aircraft carries a laser beacon and a second aircraft carries an optical detection device designed to alert the pilot of a potential collision threat and provide collision avoidance information if desired.
2. Description of the Prior Art
Studies by both the Federal Aviation Administration and the United States Air Force have shown that well over 90% of general and commercial aircraft mid-air collisions have occurred under visual flight conditions. Most of these collisions occurred between two general aviation aircraft flying during the daytime in good weather. When the weather is poor, fewer aircraft are in the air, and most of those are flying under instrument flight regulations (IFR). Although IFR aircraft are spaced from one another, collisions with aircraft flying under visual flight regulations (VFR) in marginal weather conditions or between weather systems also constitute a serious threat. Therefore a device to provide proximity warning and/or collision avoidance information to planes flying either VFR or IFR in weather conditions equal to or better than VFR flight minimums (three mile visibility) would have a strong impact on aircraft protection from mid-air collisions.
For years, flashing lights have been used for both aerial and marine navigation, providing the observer with a method for determining the bearing to the potential hazard. Most aircraft are now equipped with bright flashing strobe beacons as well as less intense red flashing beacons and various navigational lights as visual aids to avoid collisions. The effectiveness of such aircraft lights depends on the ability of the pilots of other aircraft to see them. Therefore, they are essentially of no use during the daytime. Furthermore such lights are ineffective when they are out of a pilot's field of view or when the pilot is otherwise coupled and is not looking out of the cockpit.
Electronic means for detecting such beacons have been considered by Campanella, U.S. Pat. No. 3,652,981, and by Leigh and Richardson, Proceedings of the IEEE, Volume 58, page 462. In both of those disclosures a detector or detector array is mounted so that a high intensity flashing strobe light from another aircraft can be electronically sensed. Collision probability in both of these cases is determined by the bearing of the threat aircraft relative to the aircraft with the detector system herein designated the protected aircraft. Although these inventions alleviate the need for visual contact by the pilot, they are seriously limited in daytime operation by the sun and reflected sunlight from clouds or terrain. This limitation occurs because the strobe emits light over a broad spectral range so effective spectral filtering against sunlight is not feasible. A further limitation of these inventions is their unique dependence on the bearing of the threat aircraft from the protected aircraft for determination of collision probability. Other such parameters as range, heading of the threat aircraft with respect to the protected aircraft, and time to collision cannot be determined. Large "false alarm" indications may subsequently occur.
A narrow spectral width laser navigational beacon is described in U.S. Pat. No. 3,710,098 for Walden. That beacon configuration was designed to enhance visual observation of the flash at a distance. A horizontally collimated beam and narrow spectral width are features which strongly enhance the electronic detectability of a beacon in the presence of background daylight.
A light beacon can be constructed to yield the heading of a threat aircraft with respect to a protected aircraft. When coupled with bearing to the threat aircraft, this information would further enhance the ability of such a system to predict collision probability. Greenlee et al, U.S. Pat. No. 3,903,501, discloses a modified aircraft strobe light system for providing a distinctive flash pattern in the forward direction so other aircraft will know that the beacons are mounted on an aircraft headed toward them. Nerberg, U.S. Pat. Nos. 3,609,677, Jorgensen et al, 3,721,950, Jorgensen et al, 3,662,180, and Johnson et al, 3,799,675, describe flashing beacons which, by time separation or intensity variation between two observed pulses, indicate the bearing of the observer from the beacon. In addition, Trageser et al, U.S. Pat. No. 3,846,746, discloses a strobe light based collision avoidance system in which the pulse repetition rate is actively varied to indicate the altitude and/or heading of the threat aircraft. Pulse time separation measurements systems run into difficulty when several threat aircraft fall in the same detected region. In such a circumstance the electronic observation of the various flashes may yield spurious results. Intensity variation measurements between two pulses also cannot easily cope with more than one aircraft within the observed region. Furthermore, intensity variations naturally occur due to atmospheric fluctuations, so further complications in information processing arise as is discussed by Johnson et al. In all cases, the beacons described are of broad spectral width and are therefore subject to substantial sunlight interference in daylight operation.
Several inventors have proposed RADAR-like optical devices for collision avoidance or position location. Daly, U.S. Pat. No. 3,620,626, and Proceedings of the IEEE, Volume 58, page 456, disclose such a system operating with a pulsed laser transmitter and detector mounted on the protected aircraft. Threat aircraft carry retroreflectors to enhance the reflected signal intensity. The time between pulse transmission and reception yields the range, and bearing is determined by the direction from which the reflection came. A second pulse provides range rate and redundancy. A similar system for measuring range and position is disclosed by Helm et al, U.S. Pat. No. 3,802,780, wherein a pulse modulated light source mounted on one body is reflected from another and detected on the first body. Intensity variation yields position and time delay yields range. Lecroy, U.S. Pat. No. 3,897,151, discloses a laser miss distance indicator which again uses the round trip time of light pulses to determine the position and trajectory of a missile. Skagerlund, U.S. Pat. No. 3,720,468, also discloses a reflective ranging device.
For use in aerial collision avoidance the foregoing RADAR-like optical devices require retroreflectors on other aircraft in order for signals to be large enough to permit detection over a reasonable range. Therefore, even though they have the advantage that each protected aircraft need not rely on other aircraft beacons, all aircraft must have retroreflectors. Furthermore, the round trip signal loss is large enough that the transmitted pulse must be of substantial energy and may present an eye hazard to ground personnel or nearby aircraft. These systems also cannot determine the heading of the threat aircraft and therefore must rely only on range and bearing information for collision threat determination.